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1. Introduction
Laser-based powder bed fusion (PBF) additive manufacturing (AM) is a rapidly growing technology in which complex, metal components are produced directly from digital instructions without the need for complex tooling. In recent years, AM has seen increased interest from industry and government alike (Wohlers et al., 2021). Unfortunately, flaws are still common in PBF parts (Aboulkhair et al., 2014; Gong et al., 2014; Snow et al., 2020). Since the process requires thousands of individual laser material interactions, any minor process perturbation can cause flaw formation. Fortunately, many flaws can be avoided through exhaustive processing parameter selection, typically including laser power, scan speed and spot size (Kempen et al., 2015; Leicht et al., 2020; Majumdar et al., 2019; Marrey et al., 2019; Shipley et al., 2018). Even though these parameters are optimized, and systematic flaws are reduced, there is still a potential for random, stochastic flaw formation (Bauereiß et al., 2014; Iebba et al., 2017; Schwerz et al., 2021) as well as process shifts (e.g. variation in shielding gas cross-flow due to clogged filters). To this end, in situ sensors are helpful to monitor process fluctuations and deviations(Foster et al., 2015; Lane et al., 2016; McCann et al., 2021).
Within laser-based PBF build chambers, inert gas flows across the powder bed surface (i.e. cross-flow) removing process ejecta and ensuring that a minimal oxygen environment surrounds the melt pool (Anwar et al., 2019; Bidare et al., 2018; Jakumeit et al., 2021; Ladewig et al., 2016; Philo et al., 2017). Moran et al. (2021) suggest that gas flow can have as large of an effect on the deposition process as hatch spacing, build height and component geometry. Too high of a gas cross-flow speed can disturb the powder bed and blow away a portion of the powder (Shen et al., 2020). Too little cross-flow speed will not sufficiently remove the plume and ejecta, thus leading to unstable processing (Shen et al., 2020). Therefore, most manufacturers define a flow speed that is as high as possible without disturbing the powder bed.
Measurement of gas cross-flow has been attempted using a number of methods, which are...